This application claims the priority of German patent document 100 19 772. filed Apr. 20, 2000, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a device for compensating for fluctuations in the concentration of a gaseous constituent in the fuel gas stream of a dynamically operated fuel cell system.
In the dynamic mode, during a hot start or in the event of malfunctions, concentration fluctuations in the fuel gas stream occur in fuel cell systems both upstream and downstream of the anode of the fuel cell. In the case of excessive carbon monoxide concentrations, these fluctuations lead to poisoning of precious metal catalysts in the fuel cell or, in a gas cleaning stage, to selective catalytic oxidation of the carbon monoxide. In the event of fluctuations in the steam concentration, the equilibrium in the water gas shift reaction may be displaced or condensation problems may occur.
Considerable deviations in the hydrogen concentration in the fuel cell exhaust gas ultimately lead to temperature fluctuations in a downstream catalytic burner and possibly in the reactors heated using waste heat from the burner. Fluctuations in concentration of this type are therefore undesirable, since they restrict the system dynamics or increase the control outlay. Moreover, they may destroy components in the fuel cell system or necessitate additional components.
British patent document GB 2 268 322 A discloses a fuel cell system having a reforming reactor and a fuel cell, with a hydrogen store arranged between them. The hydrogen store contains a proton-conducting membrane which on one side is exposed to the fuel gas stream of the fuel cell system and on the opposite side delimits a space which is filled with a hydrogen storage alloy. Application of a voltage to the proton-conducting membrane, causes hydrogen to be pumped through the membrane as a function of the partial pressures prevailing on the two sides. The desired pressure difference can be preset by selecting the voltage applied. Therefore, depending on the partial pressure in the fuel gas stream, hydrogen is taken out of the fuel gas stream into the storage alloy or out of the storage alloy into the fuel gas stream.
It is an object of the invention to provide a device which easily and reliably compensates for fluctuations in concentration in the fuel gas stream of fuel cell systems.
This and other objects and advantages are achieved by the apparatus according to the invention, in which a buffer component containing a material for adsorption or absorption of the gaseous constituent (with a take-up capacity which is dependent on the partial pressure) is arranged directly in the fuel gas stream, upstream of a component which is to be protected. In this way, it is easy to reduce the impact of (or at least smooth fluctuations in) concentration in the fuel gas stream of the fuel cell system. If the concentration falls below a preset level, the buffer component accordingly acts as a source, while if the concentration exceeds a preset level the buffer component acts to reduce the concentration. A further advantage of the device is that it can be operated passively, i.e., without additional control means and the associated actuators.
In an advantageous embodiment, it is also possible for the buffer component to have a plurality of partial components which are optimized with regard to individual gas constituents which are to be stored. It is therefore not necessary to provide a separate buffer component for each gaseous constituent, which leads to advantages with regard to the space required, the weight and the costs.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.